When a load is to be powered under a fixed D.C. voltage, a voltage-setting element, typically a zener diode connected in parallel with the load, is often used. A current source is then series-connected with this parallel association and the assembly is powered with a D.C. voltage. The zener diode has the function of limiting (setting) the voltage across the load. The current source has the function of absorbing the current flowing through the load and through the zener diode.
For a fixed load (constant power consumption), the current source is sized according to this current to guarantee that the zener diode is permanently in avalanche (active as a voltage limiter).
However, for a load having a power consumption which is capable of varying, the current source must then be sized according to the maximum power consumption of the load. This generates an unnecessary power consumption when the load does not draw this maximum current.